CELL MEMBRANE AND TRANSPORT ACROSS MEMBRANE
Functions of Biological Membranes
1) Definition of cell’s boundaries
● Cell membrane keeps interior of cell separated from surrounding environment
● Phospholipid bilayer is selectively permeable; allows for desirable substances to be kept
within and undesirable substances kept out of cell
2) Organisation and localisation of function
● Molecules/structures with specific functions are embedded in membranes or localised within
organelles (e.g. electron carriers in thylakoid membrane and inner mitochondrial membrane;
hydrolytic enzymes within lysosomes)
● Serve to organise and compartmentalise function within eukaryotic cells (i.e. provide
different local environments for different reactions)
3) Regulation of cell’s contents
● Proteins and other components of membrane helps regulate the transport of substances into
and out of cell/organelles (e.g. to take up and accumulate useful substances such as water,
ions and glucose into various compartments; to remove metabolic waste products; to confine
certain chemicals within specific regions of cell)
4) Signal transduction
● Specific protein receptors on outer surface of cell membrane play a key role in detection of
specific signals (after binding to receptors), and thus trigger specific responses within cell
(e.g. drug/hormone induced responses)
5) Cell-to-cell communication
● Membrane proteins bind extracellular matrix/cell surface constituents to mediate adhesion
and communication between adjacent cells
Fluid Mosaic Model
● Membrane is viewed as (a) a ‘mosaic’ or collage of proteins randomly distributed in/loosely
attached to (b) a fluid phospholipid bilayer which is free to move about laterally
● Characteristics:
○ Fluid layer is asymmetrical (two lipid layers may differ in composition and
arrangement of proteins and lipids)
○ Phospholipid bilayer is fluid/mobile; lateral movement of phospholipids
○ Unit membrane is a dynamic structure; embedded proteins can float, some moving
freely while others fixed in positions by microfilaments on cytoplasmic face
○ Membranes are amphipathic (hydrophilic phosphate head of phospholipids face
outwards into aqueous environment inside/outside cell; hydrophobic hydrocarbon
tails face inwards and create a hydrophobic core)
Phospholipids
● Amphipathic; results in formation of phospholipid bilayers in an aqueous environment
● Long hydrocarbon chains of fatty acids form an effective hydrophobic barrier against polar
and charged substances
Copyright © 2019 tonyndr
Functions of Biological Membranes
1) Definition of cell’s boundaries
● Cell membrane keeps interior of cell separated from surrounding environment
● Phospholipid bilayer is selectively permeable; allows for desirable substances to be kept
within and undesirable substances kept out of cell
2) Organisation and localisation of function
● Molecules/structures with specific functions are embedded in membranes or localised within
organelles (e.g. electron carriers in thylakoid membrane and inner mitochondrial membrane;
hydrolytic enzymes within lysosomes)
● Serve to organise and compartmentalise function within eukaryotic cells (i.e. provide
different local environments for different reactions)
3) Regulation of cell’s contents
● Proteins and other components of membrane helps regulate the transport of substances into
and out of cell/organelles (e.g. to take up and accumulate useful substances such as water,
ions and glucose into various compartments; to remove metabolic waste products; to confine
certain chemicals within specific regions of cell)
4) Signal transduction
● Specific protein receptors on outer surface of cell membrane play a key role in detection of
specific signals (after binding to receptors), and thus trigger specific responses within cell
(e.g. drug/hormone induced responses)
5) Cell-to-cell communication
● Membrane proteins bind extracellular matrix/cell surface constituents to mediate adhesion
and communication between adjacent cells
Fluid Mosaic Model
● Membrane is viewed as (a) a ‘mosaic’ or collage of proteins randomly distributed in/loosely
attached to (b) a fluid phospholipid bilayer which is free to move about laterally
● Characteristics:
○ Fluid layer is asymmetrical (two lipid layers may differ in composition and
arrangement of proteins and lipids)
○ Phospholipid bilayer is fluid/mobile; lateral movement of phospholipids
○ Unit membrane is a dynamic structure; embedded proteins can float, some moving
freely while others fixed in positions by microfilaments on cytoplasmic face
○ Membranes are amphipathic (hydrophilic phosphate head of phospholipids face
outwards into aqueous environment inside/outside cell; hydrophobic hydrocarbon
tails face inwards and create a hydrophobic core)
Phospholipids
● Amphipathic; results in formation of phospholipid bilayers in an aqueous environment
● Long hydrocarbon chains of fatty acids form an effective hydrophobic barrier against polar
and charged substances
Copyright © 2019 tonyndr
